A flat panel display includes a passive matrix (PM) type and an active matrix (AM) type using a MOS (Metal Oxide Silicon) thin film transistor (TFT). In the active matrix (AM) type flat panel display, a thin film transistor and a capacitive element are connected to respective ITO (indium tin oxide) pixel electrodes for keeping voltages by capacitance of capacitors. The active matrix type flat panel display is widely used according to tendency of high resolution.
The thin film transistor used in the active matrix type flat panel display is generally classified into a poly-silicon thin film transistor and an amorphous silicon thin film transistor according to a kind of an active layer. The poly-silicon thin film transistor has high electron mobility, excellent optical stability and reliability compared than the amorphous silicon thin film transistor. Thus, the poly-silicon thin film transistor is usually used in the active matrix type flat panel display.
Methods of crystallizing the amorphous silicon into the poly-silicon include a solid phase crystallization method, an excimer laser crystallization method capable of low temperature crystallization, a metal induced crystallization and a metal induced lateral crystallization. The solid phase crystallization method has problems that its process time is too long and a substrate is easily deformed by heat treatment at a high temperature for a long time. The excimer laser crystallization method capable of low temperature crystallization has disadvantages in that it requires an expensive laser device and an interfacial property between a semiconductor layer and a gate insulation film is bad due to extrusions formed on the poly-crystallized surface. When the metal induced crystallization or metal induced lateral crystallization is used, metal catalysts remain in a crystallized silicon layer, thereby increasing leakage current of the semiconductor layer of the thin film transistor. Various crystallization methods have been proposed in addition to the crystallization method. However, most of newly proposed methods have a problem that the leakage current of the thin film transistor is very high. Thus, it is very difficult to use the methods actually in products.
The pixel circuit of the active matrix type flat panel display should keep a data voltage during one frame before a new data voltage is applied after the pixel circuit is applied with the data voltage. In this time, the leakage current of the thin film transistor can be changed according to a change amount of the data voltage due to the thin film transistor generating the leakage current when the new data voltage is applied to a drain of a switching element. In addition, even if the same data voltage is applied to the drain of the thin film transistor, image quality degradation phenomenon such as change of brightness and degradation of image quality is caused by difference between leakage currents of each pixel.